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Show that if available carpentry hours remain between 60 and 100, the current basis remains optimal. If between 60 and 100 carpentry hours are available, would Giapetto still produce 20 soldiers and 60 trains?

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We are to show that if available carpentry hours remain between 6060 and 100,100, the current basis remains optimal. Also we are to check if Giapetto would still produce 2020 soldiers and 6060 trains. Ofcourse, maximizing the objective function is maxz=3x1+2x2,\max z=3x_1+2x_2, where x1x_1 and x2x_2 are numbers (nonnegative) of soldiers and trains produced per week, respectively. It is subjected to previous constraints:

2x1+x2100,x1+x280,x140.2x_1+x_2\leq100,x_1+x_2\leq80,x_1\leq40.

Let us b2b_2 be the number of carpentry hours available. Obviously the change in b2b_2 shifts the carpentry constraint parallel to its current position. However, as long as both constraints intersect in the feasible region the current basis will remain optimal. From the figure preented below we see that if b2<60b_2<60 intersection of finishing and carepntry constraint is not in the feasible region, thus the current basis is not optimal. We see that for b2=60b_2=60 the point (40,20)(40,20) is intersection of all 33 constraints (in feasible region ofcourse). If however b2<60b_2<60 the demand constraint would not be satisfied. If b2>100b_2>100 then finishing and carpentry constraint would never intersect, thus the current basis would not remain optimal. So the current basis will remain optimal if 60b2100.60\leq b_2\leq100. However, the optimal solution will change. Let us b2=80+Δ.b_2=80+\Delta. The current basis remains optimal if 20Δ20.-20\leq\Delta\leq20. Thus it has to be:

2x1+x2=100,x1+x2=80+Δ,2x_1+x_2=100,x_1+x_2=80+\Delta,

since we want to find a point where the constraints are binding. Subtracting the second from the first equation yields

x1=20Δ,x2=60+2Δ.x_1=20-\Delta,x_2=60+2\Delta.

Simply, increasing available carpentry hours decreases at the same rate the optimal number of soldiers produced. Similarly the number of trains increases two times the number of additional carpentry hours. Therefore, we are done.

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